1. Field of the Invention
The present invention relates to a tape speed sensor which detects a tape travel speed and a phase in a tape recording/reproducing apparatus or the like and which may correct an error signal caused by a deviation inherent in the tape speed sensor itself, a control circuit using this tape speed sensor and a method of controlling a tape speed sensor by using this control circuit.
2. Description of the Prior Art
FIG. 13 is a diagram showing a rotary encoder according to the prior art.
A rotary encoder 1 shown in FIG. 13 is for use with a tape recording/reproducing apparatus or the like without a capstan, for example, and a rotary shaft 2 of the rotary encoder is rotated as a tape is transported.
The rotary encoder 1 comprises a rotor 3 secured to the rotary shaft 2 and a stator 4 partly opposed to the rotor 3. The rotor 3 has slits 6 which comprise light-passing areas and light-shielding areas alternately formed on a circumference like a bar code. The stator 4 is shaped as a fan having slits partly defined thereon. The slit 6 may coincide with the slit 5 formed on the stator 4 when the rotor 3 is rotated.
A light-emitting diode (LED), for example, or the like is used as a light-emitting element 7, and a photodiode (PD) or the like is used as a light receiving element 8. Light emitted from the light-emitting element 7 is passed through the slits 5, 6 defined on the rotor 3 and the stator 4 and detected by the light receiving element 8, thereby being converted into an electrical signal (envelope signal). A rotation speed of a rotor and a phase displacement can be detected by processing this envelope signal with an electrical means not shown. Thus, the tape recording/reproducing apparatus is able to control a running speed, wow or flatter or the like by feeding these detected values back to a reel motor which controls a tape travel.
In the prior-art rotary encoder 1, since the information indicating that the slits 5, 6 are opposed to each other (envelope signal) is the representative detected value which results from detecting only a part of the whole of the slits 6 defined on the rotor 3, the envelope signal is low in signal level and has much noises so that it is not high in accuracy. Also, to increase the accuracy of the envelope signal, assemblies such as slits have to be finished with a high accuracy and also have to be assembled with a high accuracy. In addition, the semiconductor laser (LD) or the PD of high sensitivity is used as a light source to receive light. There is then the problem that the manufacturing cost increases extremely.
Further, when the shafts of the rotor 3 and the rotary shaft 2 are deviated from each other, an eccentricity occurs in the rotation of the rotor 3. Consequently, the detected value is such one that an error component that the rotary encoder 1 itself contains is superimposed upon the tape running speed and the wow and flatter detected by the rotary encoder 1.
However, since the conventional rotary encoder 1 cannot eliminate the error component contained in the rotary encoder 1 itself, the rotary encoder cannot obtain a more. accurate speed and cannot reduce the wow and flatter. Also, even when the encoder itself can be manufactured with a high accuracy, the wow and flatter cannot be eliminated completely. Further, even though the tape speed is controlled by directly accelerating or decelerating the rotation speed of the reel motor, the wow and flatter cannot be eliminated from a standpoint of a delay of a responsiveness of a mechanism system or a standpoint of ability of assembly.
Furthermore, in order to solve the above-mentioned problems and to make the rotary encoder 1 become high in accuracy, the prior art attaches importance to the standpoint of the structure and seeks after the finished accuracy of each assembly and the assembly accuracy. There is then a limit upon reduction of the manufacturing cost.